1. Field of the Invention
The subject invention relates to telecommunication systems, notably telephone exchange systems, wherein subscriber stations are held in, and are released from, a captive condition, such as in instances when the setup of a connection between subscribers is not completed for some reason.
2. Description of the Prior Art
In centrally controlled exchange installations, a line circuit associated with each subscriber station connects the line of the subscriber with the exchange. A subscriber station comprises a subscriber apparatus, subscriber line and line or connecting circuit. In the rest or idle condition of a subscriber station, its handset is hung up and no current flows in the associated loop circuit. If the handset is removed, the subscriber station transfers from the idle to the busy condition and a current flows in a loop circuit extending over the subscriber line. The varying electric potential conditions in the loop circuit have to be ascertained and transferred to the central control as a subscriber start indication, frequently referred to as "call", so that the central control can initiate the control operations necessary for the establishment of the desired connection, such as, among other things, the through-switching of a connection via the switching network to free or idle selector equipment.
A switching arrangement of the above mentioned type is known from the publication "Informationen Fernsprech-Vermittlungstechnik" [literally: "Information on Telephone Exchange Technology"], Issue 1, 1966, pages 3 et seq., published by Siemens & Halske AG, Berlin and Munich, and containing the article by Andreas Friese and Walter Strobelt entitled "Zentralisierte Programmsteuerung in Nebenstellanlagen ESK 400 E nach dem Siemens-Crosspoint-System" [literally: "Centralized Program Control in Private Branch Exchanges Type ESK 400 E According to the Crosspoint System of Siemens"]
In such a switching arrangement, subscriber stations may be monitored with respect to their conditions of operation without need for a provision for each subscriber station, such as in each line circuit, of switching means, such as line relays, which themselves would monitor, recognize and differentiate between the diverse possible operating conditions of the particular subscriber station or of the particular subscriber line circuit. For this purpose, the closure of the subscriber line loop of the particular subscriber station is communicated to a subscriber station finder or lead finder which pertains to a central control and which comprises the mentioned indicator station, the selector circuit and further switching means as shown in the cited article. The subscriber station or lead finder thus has the task of recognizing the closure of the subscriber loop at one of the subscriber stations and to indicate such closure to the central control. The finder relieves the subscriber line connection circuits of this task, and these circuits, therefore, need not be equipped with circuit components for evaluating operating condition variations of subscriber stations. Closure of the subscriber line loop causes the finder equipment individually to identify the particular subscriber line connection circuit and to issue a corresponding result of the identification to the central control.
The latter effects through-switching of the connection from the particular subscriber line connection circuit via the switching network of the exchange to free or idle selector equipment. Through closure of the subscriber line loop an up-to-then idle subscriber line connection circuit thus transfers to the call condition; through the call condition the station or lead finder is excited; and the central control becomes eventually active after performance of the station finder process.
The German patent specification No. 1 265 222 discloses the captive condition for subscriber line connection circuits. In correspondingly equipped telecommunication exchange installations, a subscriber line connection circuit reaches the captive condition if an existing connection is released from the part of another subscriber station which up to then was connected to the subscriber station of the particular line connection circuit, and if therefore the subscriber loop in that station is not yet opened. Also, a subscriber line connection circuit may reach the captive condition in case of premature release of a connection being in the establishment process, as well as in case of erroneous dialling, excessive delays between dial actuations or in case of a busy condition of the called subscriber.
The circuit components arranging the captive condition in the line circuit ascertain that the setup of a connection from or to the particular subscriber station is only then again possible, when the subscriber has hung up his handset. In this manner, the exchange is enabled to clear equipment and lines without the subscriber having to hang up his handset and without an as such completely unintended renewal call taking place thereby, which would mean a use of the central control in addition to the station finder equipment.
Although the subscriber station is separated from the exchange, the loop circuit is, however, closed, as long as the handset of the subscriber station has not been hung up; the subscriber station then being in effect busy. As soon as the handset is hung up, the subscriber station transfers from the busy to the idle condition. This change in operating states of the subscriber station also has to be ascertained, so that the conditions applicable to the captive state of operation of the subscriber station may be cancelled.
This is of particular importance in exchanges in which, in connection with a catching circuit in the subscriber station, further operative features, such as, for instance, the feed-in of a busy signal into a subscriber station "caught" after a release, are provided. The feed-in of a busy signal into a subscriber station in the captive condition enables the particular subscriber to recognize the captive condition immediately and causes him to hang up his handset prior to initiation of a new dialling operation (call, dialling of a number).
The mentioned German patent specification No. 1 265 222 discloses a circuit arrangement wherein the captive condition is monitored by circuit components individually provided in each subscriber line connection circuit. For this purpose, there is provided a relay which is held energized by a holding circuit extending via the particular subscriber line loop and which drops out as soon as in the subscriber station the particular subscriber line loop, and thus the holding circuit of that relay, is opened for the purpose of a release of the subscriber line connection circuit.